This invention relates to liquid hydrocarbons and methods for producing them and more particularly to high thermal stability liquid hydrocarbons and their methods of production.
As the Mach number of supersonic aircraft increases, the air-frame skin temperature and engine inlet temperature increase rapidly. The net result is that the fuel used to power the aircraft is exposed to greater and greater thermal stress as the speed of the aircraft increases. If the fuel fails under thermal stress the aircraft can be rendered inoperable in a variety of ways. For example, degraded fuel can form deposits and sediments which can markedly lower heat transfer coefficients in key areas and/or plug narrow tolerance parts and filters. For a high speed airplane operating at mach 4.5, ram air temperatures are in the range of 1400.degree. F. In such situations, the fuel is the only material present which can be used as a heat sink for cooling.
Present day aircraft turbine engine fuel does not possess the thermal stability necessary to satisfy the requirements of a Mach 4 to 5 aircraft. In the past, a number of proposals have been made for providing a high thermal stability jet fuel, but these proposals each have drawbacks. For example, it has been proposed to use specialty fuels such as methylcyclohexane, but these fuels are extremely high in cost and are not readily available. Also, it has been proposed to use cryogenic fuels, but such fuels are impractical because of the low temperatures handling problems and the high fire and/or explosion hazard involved with use of H.sub.2 or CH.sub.4 as a fuel in an aircraft. Also, attempts have been made to produce fuels for high speed aircraft by making major changes in the physical composition of present day fuels, but such high speed fuels could not be used interchangably in lower speed aircraft.
In the past, there have been studies on the factors that affect the high temperature properties of hydrocarbon fuels. For example, an article coauthored by Thomas J. Wallace and myself, entitled "Kinetics of Deposit Formation from Hydrocarbon Fuels at High Temperatures", and appearing at pages 258 to 262 in Vol. 6, December, 1967, of I & EC Product Research and Development, discloses that molecular oxygen adversely affects fuel stability. The article also discloses that trace levels of sulfur compounds influence the deposit formation process, that olefins may adversely affect stability and that high temperature deposits contain higher sulfur and oxygen contents than the base fuel while low temperature deposits contain higher sulfur, oxygen and nitrogen contents. The article, however, is primarily concerned with aircraft fuels for a Mach 2.7 aircraft and temperatures on the order of about 500.degree. F. and does not disclose how to produce a thermally stable fuel nor a fuel that can be used at higher temperatures nor the effects of trace compounds on deoxygenated fuels. Similarly, an article by A. C. Nixon and H. T. Henderson, entitled "Thermal Stability of Endothermic Heat-Sink Fuels", and appearing at pages 87 to 92 in Vol. 5, March, 1966 of I & EC Product Research and Development, discloses that deoxygenation will improve fuel stability. This article, however, is not concerned with the effects of trace impurity compounds such as sulfur and nitrogen compounds and primarily is concerned with pure hydrocarbon compounds. Previous work on deoxygenated jet fuels often produced erratic results in that thermal stability was improved in some cases but not in others and offered no clue as to why one fuel would improve in stability with deoxygenation and another would not. As a result, deoxygenation has not been generally accepted as a reliable method for improving jet fuel stability.